Nice video!

Do you remember what the range was?

I've seen the flash from a similar 3"x5" US military signal mirror at up to 44 mile range, though at that range it looks more like a real star (point source) than the dazzling flash you see at closer range here.

Those 3"x5" signal mirrors are still made for the US military by S.I. Howard Glass, and sold to civilians through distributors - if you want one, Google for MIL-M-18371E. That's pretty much the only decent 3"x5" glass signal mirror sold in the USA today (don't buy the lookalikes with non-functional aiming grids - see Doug Ritter's warning article: Warning: Phony Signal Mirrors to see how to spot those.)

Note that the S.I. Howard MIL-M-18371E signal mirrors come in both a 2"x3" and 3"x5" size, so make sure you get the size you want. In the 2"x3" size, the Coghlan's Survival Signal Mirror is more widely sold, much cheaper, and pretty much as good (as long as you don't leave it under water - that rapidly penetrates to the aimer and fogs it up - for a while they had that problem licked, but units I've tested recently, leak),

There's a Boy Scout event called Operation On Target where teams get up on peaks to signal each other with signal mirrors. We mainly use foot-square signal mirrors, but I always try to see how far we can see 3"x5" and 2"x3" mirrors as well.

Operation On-Target was held in pretty much every Western state for decades (including Colorado), but since the LDS pulled out of Scouting, we've just been hanging on in Southern California.

On the "Operation On-Target" web site, we have a nice resource page on signal mirrors:
Making Your (own signal) Mirrors with DIY instructions and links to articles about the evolution of US signal mirrors in World War II.

My YouTube channel: rafowell YouTube Channel is mainly devoted to signal mirrors and related topics.